Men of Iron

They were apparently in something.

The Men of Iron were sentient, sapient robot servants created by humanity during the Dark Age of Technology. They were companion systems to the Standard Template Constructor: the STC processed data to generate plans and blueprints (such as those of the Men of Iron), which the Men of Iron would execute and build (though some STC systems could build things on their own).

Unfortunately, they were capable of learning and self-improvement; before long, they were smart enough to realize that the squishy organics were dependent on them. This so insulted them that they rebelled against humanity. They were eventually put down, but the rebellion, along with the other dangers of the galaxy, was enough of a disruption to send humanity into an Age of Strife.

Few records remain of this time in history, so the form that these Men of Iron took is not known, but their rebellion still left a huge impression on the nascent Adeptus Mechanicus; even to this day, they strictly forbid the creation of "abominable intelligences" and shun even the idea of self-improving machines, preferring to use relatively tame servitors instead.

Some Men of Iron still survive to this day, locked up in ancient pre-Imperial technology archives. Because tech-priests can't keep their noses out of ancient technology, these archives are often inadvertently activated. The Tanith First and Only also discovered an STC factory that had been corrupted by Chaos to produce Men of Iron, and they destroyed it immediately thereafter.